I have 2 sheets of plywood approx 6ft x 18" x 3/4" and my plan is to use them as bench tops in a shed. I need to cut them cross ways so they fit in my car for transportation.

What is the best way to do this so they can be re-joined easily later on?

I was thinking if I angled the blade like the third cut in this image then it would afford more support for whatever is resting on the bench,
instead of using a simple perpendicular cut like the first.

Unless you're driving a convertible, bodge together some temporary roofracks: strips of thick styrofoam (between the roof and the ply) and straps through the passenger compartment. One more strap running fore and aft to prevent uplift at the front edge. Add a couple of fasteners between the sheets to keep everything from slipping. And keep the speed down.
– Aloysius DefenestrateFeb 6 '16 at 15:20

Unfortunately this wouldn't work for me. The journey is a 1.5 hour motorway drive (120km/h) or a 2.5 hour drive by national roads (80 - 100km/h). I wouldn't have confidence in doing that journey with planks of wood somewhat slapdashedly affixed to the roof. Anyways, this being the Woodworking stack, It'd be more fun to make the wood fit the car than the car fit the wood :)
– cdslnFeb 10 '16 at 21:01

Buy the plywood locally instead. That will cost less than the fuel for your journey.
– James YoungmanOct 28 '16 at 19:58

3 Answers
3

In terms of how to cut the board apart I don't think it matters that much, but with an angled cut providing a wider glue surface area that will provide a stronger joint. In the right context I doubt this increased strength will actually matter though, since I assume the sheets will be supported underneath when installed as your worktops.

If the plan was to have the glued-up boards support themselves and then any weight put on top IMO you'll want to rethink. Even a very shallow cut giving a scarf joint I doubt would provide the needed strength if unsupported.

I think your major difficulty is going to come when it comes time to glueing the pieces of ply back together, you would ideally want to have at least three long clamps with an opening somewhat wider than 6'. Obviously clamps this size don't come cheap. One option is to make rudimentary sash clamps from 2x4s, each would be nearly seven feet long but could be cut down to a more manageable length after the worktops are assembled so they would then be more practically usable in the shop. However transporting that length of 2x material might present you with the same problem as with the plywood in the first place!

An alternative method to allow clamping that is sometimes used is to attach temporary glue blocks to the wood, similar to how mitre joints are sometimes held together. These blocks can be directly glued on or attached with an intermediate material (often paper), see tip here for a little more, however with plywood instead of solid wood it might not work as the surface ply can be weak. However, unlike when doing mitres here you have an advantage as one surface will be hidden so instead I would suggest screwing clamping blocks directly to the underside of the plywood, these can simply be unscrewed after the glue has cured.

The small holes left in the ply after the screws are removed won't harm its strength.

Unless I made a mistake converting dimensions, this is a mere 45*182cm, and 19mm strong. Which will, as LarryBud said, fit in ANY car, yours included. As for leather seats, simply putting a bedspread or such in between wood and seats will do fine (I do that when I drive wood, but usually with pieces that are 250cm rather than 180).

But...

... it seems that you really want to cut that sheet in half, so... if you insist on doing that, I would do a straight cut. Not only is this easiest, but you can also use a couple of dowels, which greatly adds to stability and prevents you from going insane trying to glue those slippery bastards together in a straight way. Dowels are no fun with a slanted edge, though.

I recommend fish dowels (3 or 4) in this case (Edit: I just realized that the English word for these is "biscuits"), for which you can conveniently and extremely precisely cut the slit using the circular saw that you have (no free-hand drilling with an improvised jig into end grain as with ordinary dowels... what a nightmare!). That will ensure that the pieces fit together snugly and don't give you trouble shifting around while glueing. Plus, the dowels by themselves are surprisingly stable already.

The thing I'm talking about looks like this:

There are specialized litte joining saws for that, but you can do just fine wirh a normal circular saw. Simply adjust the parallel guide on your saw table so the cut lands approximately in the middle, and then you cut a notch into both pieces that are to be joined using the same setting. Voilà, identical matching notches, plug in fish dowel and add glue, press and done.

For a more favorable looks, be sure not to cut the notch all the way through, instead stop 2-3cm before the edge.

Back when I was in uni, I made a desk that went around the room's corner from a hardboard plate (like what people uses as work surface in the kitchen) joined with a couple of fish dowels and held together with a pair of screw connectors. No glue, just the dowels. Lasted for years, and it would support, well, a female student sitting on it, without breaking. No problem.

Unfortunately I don't have access to ANY car - just my own, and it certainly doesn't fit in there. Besides the point that the seats are leather and I wouldn't want to risk damaging them by way of rough timber. Freehand cutting wouldn't pose any problem (a less than straight perpendicular cut will be fine) - as long as I cut in a single run - it will glue back uniformly. Small projects like this are enjoyable for me as is getting the chance to use my new circular saw :)
– cdslnFeb 10 '16 at 20:55